To date, organic EL devices including a plurality of emitting layers each of which emits light of a different wavelength are known. Such organic EL devices are also known to provide mixed-color light in which the lights emitted by the emitting layers are mixed together.
An example of such organic EL devices includes a layered red emitting layer, green emitting layer and blue emitting layer, and provides white light in which the light emitted by the emitting layers are mixed together.
Excited states of organic compounds encompass a singlet state and a triplet state.
Emission from the singlet state is called fluorescence while emission from the triplet state is called phosphorescence. Singlet state and triplet state typically occur in a ratio of 1:3.
In typical organic EL devices, fluorescent materials for emitting fluorescence have been mainly used. According to organic EL devices that use such fluorescence emission, only the singlet (i.e., 25% of the excitation generated in emitting layers) contributes to the emission, so that triplet (i.e., the remaining 75%) is deactivated without emitting.
In order to enhance luminous efficiency of organic EL devices, developments are being made on phosphorescent materials for emitting phosphorescence, i.e., emission from triplet (e.g., Patent Document 1).
Reports have been made on, for instance, red-emitting phosphorescent materials and green-emitting phosphorescent material.
It should be noted that no practically-applicable blue-emitting phosphorescent material has been obtained.
With application of the above phosphorescent materials, organic EL devices for mixed-color emission are capable of enhancing luminous efficiency.
For instance, a known organic EL device provides white emission with use of blue-emitting fluorescent materials and red-to-green-emitting phosphorescent materials.
According to such a device, enhancement of quantum efficiency in emission of red to green color contributes to enhancement of luminous efficiency of the entire organic EL device.
In such an organic EL device of mixed-color emission, however, while emission is obtained from triplet in the red to green emitting layers, triplet in the blue emitting layer is deactivated without contributing to emission.
Accordingly, proposals have been made on an organic EL device in which triplet in the blue emitting layer is diffused in the red and green emitting layers so that red and green phosphorescence is obtained therefrom (e.g., Patent Documents 2 and 3 and Non-Patent Document 1).
According to such an organic EL device, the energy of triplet in the blue emitting layer, which has been typically to be deactivated without contributing to emission, can be utilized for obtaining red and green phosphorescence. Thus, luminous efficiency of the entire organic EL device can be enhanced.
The organic EL device disclosed in Non-Patent Document 1 includes a blue fluorescent-emitting layer, blocking layer, red phosphorescent-emitting layer, green phosphorescent-emitting layer, blocking layer and blue fluorescent-emitting layer in this order.
According to the document, blue fluorescence is obtained from the singlet in the blue fluorescent-emitting layers, and the triplet in the blue fluorescent-emitting layers is diffused in the red and green phosphorescent-emitting layers via the blocking layers.
Then, triplet in the red and green phosphorescent-emitting layers is generated, from which red and green phosphorescence is obtained.
According to the report, the blue fluorescence and the red and green phosphorescence are mixed together, thereby providing white emission as a whole.
Patent Document 1: US 2002/182441
Patent Document 2: WO2006/038020
Patent Document 3: WO2004/060026
Non-Patent Document 1: nature vol 440 p. 908